I can't manage an armoury link at the moment (silly firewalls) but the character in question is Desmona on Kael'Thas if you care to look.

The advice i'm looking for is about the gear reset transitions. When I hit 60 and start tanking Outlands instances, 70 for Wrath, and finally 80 and Cata.

Actually, the Cata ones I am not that worried about, since I am going to quest my way thru Hyjal to open Firelands and Vasjir (I think) for the tanking head enchant. By then I should have enough quest rewards to carry me thru.

What should I do at 60 and 70? Stop tanking and go quest for a level or two? Just tank it and hope my Heirlooms can carry me thru till I get some drops?

Is the transition from expansion to expansion something I should even be worried about?

I'd rather be a failure at something I love than a sucess at something I hate. All things being equal though I would rather be a sucess at something I love.

At level 70, craft or buy a full set of Cobalt armor for your non-Heirloom slots. That'll easily give you a big stat boost (especially Strength, as old tank gear pre-WotLK didn't have any, but also lots of Stamina).

60 is harder, but you can get a lot of items from the early Hellfire Peninsula quests. I'm not sure what it's like tanking those instances in pre-BC gear. You may have trouble from all the blue-geared DKs in the runs regardless of what you get.

I don't know about threat, but I never had any issues healing the lvl 58-60 tanks in BC dungeons. That being said, I don't know what exactly their gear looked like either, but as long as you aren't starting out with a fresh healer and pulling half the instance, you shouldn't have too many problems survival-wise. Take a few pulls each run for you and the healer to get accustomed to each other (especially if it's your first time tanking the instance on that toon) and you should be fine. If dps start pulling threat, any decent healer should be able to keep them healed (assuming the mob lives long enough to even damage them.)